1. Field of the Invention
This invention relates generally to a control apparatus of a low-cost multi-frequency cathode ray tube (CRT) display such as a computer monitor and, more specifically, to a control circuit of the high voltage power supply of the CRT.
2. State of the Art
The tube of a CRT display typically comprises a screen biased at the anode voltage of the tube. The cathode of the tube is provided for emitting a video-modulated beam of electrons toward the screen. Horizontal and vertical deflection yokes deflect the electron beam so that it sweeps a predetermined area of the screen. The video modulation of the intensity of the electron beam during the sweeping of the screen enables displaying a picture on the screen.
FIG. 1 schematically represents a known control apparatus of a low-cost multi-frequency monochrome CRT display 2 having an anode 4 and a cathode 6. Such a control apparatus is for example described in the “Nouveau guide de télévision en couleurs”, issued by the Syndicat de Constructeurs d'Appareils de Radio et de Télévision (SCART), pages 197–202, concerning the horizontal sweeping yoke, and in the application notes of the circuits having the reference numbers 9112 or 9112A from STMicroelectronics concerning the power supply of the yoke.
The horizontal deflection yoke 8 of the display is connected to a yoke control unit comprising a diode 12, a switch 14 and capacitors 16 and 18. The cathode of diode 12 is connected to a first end of the yoke 8. The anode of diode 12 is connected to a ground or common reference voltage. Switch 14 and capacitor 16 are connected in parallel with the diode 12. The second end of yoke 8 is connected to the ground through capacitor 18. The first end of yoke 8 is connected through an inductor 20 to a power supply 21. Power supply 21 comprises a controlled current source 22 delivering an average supply current Is. A capacitor 23 is connected in parallel with current source 22. Current source 22 is a known step up current mode source.
Inductor 20 forms the primary coil of a low/high transformer 24 of an anode biasing unit. A high turn count secondary coil 28 of transformer 24 has a first end connected to the ground and a second end connected to the anode of a diode 30. The cathode of diode 30 is connected through a low pass filter 32 to the anode 4 of the display. Low pass filter 32 is illustrated as comprising a resistor 34, corresponding to the resistance of the coil 28, connected in series with diode 30, and a capacitor 36 connected between the anode 4 and the ground.
A sensing coil 37 of transformer 24, comprising a low number of turns compared to coil 28, has one end connected to the ground and one end connected to a control circuit 38. The control circuit 38 comprises a feedback block 39 and may comprise other blocks that are not illustrated. Feedback block 39 comprises a rectifying diode 40, a filtering capacitor 42, a voltage divider and an amplifier. The anode of diode 40 is connected to sensing coil 37. The cathode of diode 40 is connected to the ground through capacitor 42. The voltage divider is comprised of two resistors 44, 46 connected in series between the cathode of diode 40 and the ground. The amplifier comprises an operational amplifier 48 having its inverting input connected to the node between resistors 44 and 46 and its non-inverting input connected to a reference voltage Vref. A capacitor 49 is connected between the inverting input and the output of amplifier 48 in a known manner for stability purposes. The output of amplifier 48 controls the current source 22. While most often the current source 22 works in a switched mode, the important point is that the average of the current it supplies is controlled by the output voltage of amplifier 48.
For sweeping a line of the screen, switch 14 is alternatively opened and closed to make the current in yoke 8 increase at a predetermined speed from a predetermined negative maxima to a predetermined positive maxima, then to make the current in yoke 8 return fast to the predetermined negative maxima (current flyback). This causes the electron beam to sweep a line of the screen and then return fast to an appropriate position for sweeping the next line. The vertical deflection yoke (not illustrated) is controlled to move the beam vertically to the next line during flyback.
A voltage peak appears across inductor 20 during flyback. A corresponding high voltage peak appears across coil 28, which is rectified through diode 30 and smoothed by filter 32 with a time constant which may be equivalent to the display time of several pictures, i.e. generally more than 10 ms, so as to obtain a substantially constant high D.C. biasing voltage Vb.
The amplifier 48 of the feedback block 39 receives a monitoring voltage Vm substantially proportional to the biasing voltage Vb, and regulates the supply current Is so that voltage Vm remains equal to the reference voltage Vref. This keeps the supply voltage Vs substantially constant, whereby the current maximum in coil 8 and the biasing voltage Vb remain substantially constant. The feedback block 39 performs as an integrator because of the capacitor 49.
The above control apparatus is known to be prone to a problem often referred to as “dynamic breathing” of the screen. The dynamic breathing corresponds to a variation of the length of lines of the screen when a sharp change of the brightness of an image being displayed on the screen occurs.
A need exists for a control apparatus that provides for the functionalities of the above control apparatus while suppressing the dynamic breathing of the screen.